(57) A position sensor has a detector (10) having a plurality of sensitive elements (12)
which are acted on by indicia (4) of a scale (2) so that each sensitive element detects
overlap between itself and one or more indicia (4) of the scale (2). The outputs of
the sensitive elements (12) are modified by weighting values so that there is one
position of detector (10). and scale (2) where the sum of the outputs has a unique
characteristic. This position can be detected by means analysing the sum of the outputs,
and can then detect displacement of scale (2) and detector (10) from that one position.
The indicia (4) used have a non-uniform pattern, but other indicia (6, 8) with a pitch
less than twice but not equal to the pitch of the sensitive elements (12, 14, 16)
may be provided to permit interpolation to fractions of the pitch of the sensitive
elements. Further interpolation may be provided by a register (40) through which signals
propagate at a rate which is a multiple of the rate they are received. The register
elements modify the signals by weighting values and the sum of the register element
outputs is determined.
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